link: take absolute path of main file (#1727)

Windows has trouble with long paths. Other paths to .a files are
absolutized already.

Related bazel-contrib/bazel-gazelle#321

go/tools/builders/link.go

@@ -25,6 +25,7 @@ import (
 	"log"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 )
 
@@ -49,6 +50,16 @@ func run(args []string) error {
 		return err
 	}
 
+	// On Windows, take the absolute path of the output file and main file.
+	// This is needed on Windows because the relative path is frequently too long.
+	// os.Open on Windows converts absolute paths to some other path format with
+	// longer length limits. Absolute paths do not work on macOS for .dylib
+	// outputs because they get baked in as the "install path".
+	if runtime.GOOS != "darwin" {
+		*outFile = abs(*outFile)
+	}
+	*main = abs(*main)
+
 	// If we were given any stamp value files, read and parse them
 	stampmap := map[string]string{}
 	for _, stampfile := range stamps {